Generally, an image printed by a thermal printing system consists of rows of dots. Together, the size of individual dots and proximity of each dot to another generate the tone of a thermally printed image. By judiciously positioning and sizing such dots on a recording medium and, consequently, controlling the image tone, a thermal printer can produce high quality images including printed characters, diagrams, pictures, and the like.
To generate printed dots, a thermal printing system employs a print head modulator and thermal printing apparatus. The modulator creates signals that cause the printing apparatus to physically print the dots upon a recording medium such as paper. In particular, the print head modulator interprets a series of data values, each of which defines the size of a dot, and then generates a modulation signal corresponding to each data value. The modulation signal controls the printing printed on the recording medium.
As is well known in the art, a typical thermal printing apparatus comprises a thermal print head, an assembly for incrementally advancing a recording medium past the head after each row of dots is printed, and an ink film disposed between the print head and the recording medium. Generally, the thermal print head contains a number of thermal head elements arranged in a linear fashion. Each of these thermal elements is individually activated by an electric current. The current, in turn, causes the activated thermal element to heat and thereby melt ink located on the ink film at a position adjacent to that thermal element. As such, melted ink transfers to the recording medium to form a line of dots where each dot in the line is associated with an activated element in the print head. Subsequently, the recording medium and the ink film are incrementally moved by a roller; thereafter, another line of dots is printed, and so on. The process is repeated until a complete image is printed on the recording medium.
The modulator controls which thermal elements within the print head are active and the amount of heat each thermal element generates. The size of each dot is proportional to the amount of heat produced by that thermal element when printing that dot. As an increasing amount of heat is generated, correspondingly increasing amounts of ink melt, producing proportionally larger dots.
The art teaches two forms of print head modulators: analog and digital. In conventional analog print head modulators, analog circuitry is used to control the magnitude of the heating current applied to each thermal element. Hence, the magnitude of the heating current controls the heat produced by a given head element and, consequently, the size of the printed dot.
In contrast, digital print head modulators, such as that disclosed in U.S. Pat. No. 4,532,523 (issued to H. Tanaka on Jul. 30, 1985) uses pulse count modulation (PCM). In general, a PCM-based print head modulator selectively applies a fixed amplitude current to each thermal element. To control the dot size, a fixed duration digital modulation signal (i.e., a current pulse) is repeatedly applied to a print head driver circuit. This circuit alternately switches current, which flows through a thermal element, on and off in response to the modulation signal. By repeatedly applying the current pulses for successive durations, the thermal element becomes progressively warmer with each such pulse. This, in turn, causes an increasingly larger amount of ink to be transferred to the recording medium and, hence, produces an increasingly larger dot.
In general, a PCM-based print head modulator converts a series of data values into a series of modulation signals. Each data value represents a dot size. The series of modulation signals are, in turn, applied to the thermal elements via print head driver circuitry. By applying heating current to a number of thermal elements in the print head, a line of dots is printed. Each printed dot is sized commensurate with the value of a corresponding data value. For example, if a data value uses 8 bits to define the size of a dot, the associated modulator can produce modulation signals corresponding to 256 dot sizes ranging from 0 to 255. Each dot size corresponds to a dot density level, also known as a gray scale level.
In a PCM-based print head modulator, the data value for a given dot is successively compared to a series of modulation values in order to produce modulation signals corresponding to each dot. The resulting series of comparisons determines the number of fixed duration pulses that must be applied to a thermal element to generate a desired dot size. Through the use of a simple form of PCM, a modulator directly compares a given data value to each one of a series of modulation values ranging from 0 to (2.sup.n -2). Whenever the data value is larger than a modulation value, a logical "1" is generated as a modulation signal. Thereafter, a current pulse of fixed duration is applied to an appropriate thermal element for each modulation signal having a logical "1" value. Since the modulation values linearly increase in value, as the data value increases, so will the number of current pulses that are applied to the thermal element. Consequently, the printed dot size will also increase as a result of an increase in the data value.
A complete thermally printed image is produced by repeating the modulation value comparisons for every data value representing an image and applying the resultant modulation signals to the printing apparatus. Unfortunately, since each data value must be compared to each of 2.sup.n -1 modulation levels, a relatively long processing time is required to generate a complete halftone image. Owing to the large number of comparisons, PCM based thermal printers generally print at quite a slow pace.
To reduce the number of comparisons required to generate each dot, and thereby increase the print speed of each line of dots, a technique known as partial pulse modulation (also referred to as "modified PCM") has been developed in the art. Specifically, U.S. Pat. No. 4,994,822 (issued to H. Caine on Feb. 19, 1991--hereinafter referred to as the Caine '822 patent), discloses a partial pulse technique in which modulating pulses of various widths are used, in lieu of fixed width pulses, to achieve each gray scale level.
Generally, a partial pulse modulator generates a series of modulation signals for each data value. The modulation signals cause the print head driver circuitry to successively apply a series of full duration current pulses (one unit in duration) plus a number of partial duration current pulses (fraction of a unit) to a thermal element. The combination of full and partial current pulses applied to such an element defines the size of a printed dot generated by that thermal element.
Specifically, a partial pulse modulator designates a number of bits within a data value as so-called "partial pulse" bits. Typically, the partial pulse bits are one or more of the least significant bits of a data value. The bits not designated as partial pulse bits are known as PCM bits. The partial pulse modulator generates the full and partial current pulses by separately operating on the PCM and partial pulse bits.
In this regard, the Caine '822 patent teaches the use of three least significant bits of a 9-bit data value as partial pulse bits. These partial pulse bits are separated from each data value. The remaining 6 most significant bits are used as PCM bits and are compared to each of 63 modulation values, i.e., 2.sup.6 -1 values. As in conventional PCM, whenever a 6-bit data value is larger than a selected 6-bit modulation value, a fixed duration pulse is applied to a thermal element associated with the data value currently being processed. The pulse duration, or pulse width, is fixed to a one unit length.
Subsequent to processing the PCM bits, the three partial pulse bits in the data value are processed individually by dedicated circuitry. In particular, the three partial pulse bits are categorized as being in positions 0, 1, or 2 corresponding to the first, second, or third least significant bit positions in the data value. Whenever a logical "1" appears in position 0, 1, or 2, a pulse 1/8, 1/4, or 1/2 the duration of the one unit pulse, respectively, is applied to the same thermal element to which the fixed duration current pulses were previously applied. In essence, the PCM bits apply integer value pulse durations to the thermal elements, while the partial pulse bits apply fractional duration pulses to the same thermal elements. Therefore, for a modulator capable of handling a 9-bit data value, 512 gray levels are achieved using only 63 modulation value comparisons. In contrast, conventional PCM would require 511 such comparisons. Since the same gray scale print is produced with far fewer comparisons, a significant increase in printing speed results with the use of modified PCM.
The minimum duration corresponding to a partial pulse bit, i.e., the pulse duration increment corresponding to each gray scale level, is limited by the minimum heat needed to generate a detectable incremental change in the size of a resulting dot. In practice, the minimum pulse width must be sufficiently long to sufficiently heat the thermal element to print a dot, while the maximum pulse width must be sufficiently short not to saturate the recording medium with ink or damage the thermal element through thermal breakdown. Each thermal print head has a thermal efficiency characteristic; this characteristic being defined as a ratio of an incremental change in thermal element drive current to the resulting change in heat so generated. The minimum pulse width is defined by both the least amount of heat necessary to melt the ink and the thermal efficiency of a thermal element. Therefore, the thermal print heads and their associated modulators must be designed sequentially, i.e., thermal element characteristics such as thermal efficiency and saturation define the minimum and maximum pulse lengths that are to be produced by the modulator, and consequently, the maximum number of partial pulse bits.
Currently, to achieve a marked improvement in printing speed by using partial pulse modulation, the modulator circuitry is designed to accommodate a specific number of partial pulse bits. Detrimentally, a different modulator design is required for each combination of data value bits and partial pulse bits. A single modulator design that can be used with any number of partial pulse bits, or none at all, is desirable to eliminate the need for a number of different modulator designs. In other words, a generic modulator is needed. Up to now, no such generic modulator exists in the art.
Therefore, a need exists in the art for a modulator useful in a thermal printing system that generates halftone images using either partial pulse modulation or pulse count modulation. Additionally, the modulator must be relatively simple and capable of accommodating any number of partial pulse bits.